The use of semiconductor lasers in optical communication systems has proven to be advantageous, particularly because of the ability to modulate higher frequencies, and low-loss signal transmissions over large transmission lengths without intermediate amplifiers.
To enable the semiconductor laser to modulate quickly, it must be supplied with a preconduction current, which is slightly above the threshold current, during the data transmission. The modulation current is then added to this preconduction current. However, any fluctuations of the threshold current caused by aging or temperature variations must be compensated by a control circuit.
The actual threshold current is usually acquired by sampling the current-voltage process of the laser diode, whose magnitude serves as the starting value for controlling the preconduction current ("Elektronik" Magazine, 22/85, p.91). The control has the special task of maintaining the maximum and minimum optical output power constant during laser operation.
To measure the optical power emitted by the laser, it is further customary, similarly to U.S. Pat. No. 4,344,173, to integrate a monitor diode into the housing of a laser diode, which produces a cut-off current in direct proportion to the optical laser output. However, the coupling ratio between laser diode and monitor diode, and thus the proportionality factor between optical power and monitor current as well, is subjected to strong specimen scatter.
For that reason, the preconduction current control of a laser diode must be adjusted before the first startup, so that the control circuit can actually capture the emitted optical power. The control takes place in the known manner, either in accordance with the optical output during laser diode operation with preconduction current, where the optical power without modulation current is adjusted to a specific value, or in accordance with the mean value of the optical output power, which occurs with continuous modulation, while the cut-off current produced in the monitor diode by the specified optical power is compared to a reference value ("Elektronik" Magazine, 22/85, p.91-92).
However, these known methods require a control to function accurately, because the proportionality factor between the optical power and monitor current is not known.